This invention relates to control systems for vehicle transmissions of the infinitely variable drive ratio hydromechanical form.
More particularly the invention provides for controlling the operation of a hydro-mechanical transmission having mechanical drive input and output sections with planetary gearing for establishing a plurality of fixed drive ratios in both forward and reverse and which also have a hydrostatic section including a pair of hydraulically coupled variable displacement fluid translating devices arranged to enable infinite variation of the drive ratio of the transmission throughout the plural forward and reverse mechanical speed ranges. By enabling infinite variation of drive ratio throughout an extremely broad range of ratios, a transmission of this kind enables selective control of vehicle ground speed and wheel torque by adjustment of the transmission itself while engine speed is maintained constant, usually at the particular engine speed at which power output per unit of fuel consumed is maximized.
A highly advantageous vehicle transmission of the general type discussed above is disclosed in detail in prior U.S. Pat. No. 3,897,697. To fully realize the advantages of such a transmission, a control system is needed which involves several control functions and implementing mechanisms having unique characteristics.
Unlike many older forms of vehicle drive systems, the operators accelerator pedal in a vehicle employing the above identified transmission does not control engine speed. Further, if the accelerator pedal were to be simply linked with the displacement adjustment of the transmission to vary drive ratio with pedal position in a straightforward manner some serious control problems would be encountered at least in certain types of vehicle. Such an arrangement would under certain conditions enable very precise control of ground speed with the accelerator pedal but this would be gained at the cost of ease of control of wheel torque. With the accelerator pedal at some given position dictating a specific drive ratio and with the engine governed to operate at a constant engine speed, any change of load or resistance to vehicle movement would be sensed by the governor which would then shift to increase or decrease engine output power and torque as necessary to maintain the constant engine speed. Wheel torque would then change accordingly although the operators accelerator pedal had not been moved. This imprecision of wheel torque control would be compounded in vehicles where the same engine powers other implements such as the bucket of an earthmoving loader. Changes of loading on those other implements also cause excursions of engine torque which could result in wheel torque fluctuations if transmission drive ratio remains constant.
Direct precise control of wheel torque is, as a practical matter, more important than direct precise control of ground speed under certain operating conditions and particularly in certain types of vehicle which may be frequently operated under those conditions. The primary concern of the vehicle operator during start-up and initial acceleration, while pushing or pulling a heavy load or while traveling on a soft or slick surface is to control wheel torque rather than ground speed as such. Too much wheel torque at such times results in a loss of traction and wheel spinning while too little wheel torque unnecessarily delays progress of the vehicle and detracts from job productivity in the case of earthmoving vehicles or the like.
Thus to ease the operators task and to gain precision in control of the vehicle, a control system for the above identified transmission should enable the accelerator pedal or the like to be used primarily as an output torque control rather than for directly regulating output speed as such or for controlling engine speed. Ease and precision of vehicle control could be still further enhanced if the range of output torques realizable between the extremes of travel of the accelerator were itself adjustable by the operator to accomodate to the different requirements of different ground conditions and loads. The operator could then set the system for very fine control within a narrow range of wheel torques or relatively coarse control within a broader range of torques to accommodate to different specific working conditions.
If the operator were provided with still another control, independent of the accelerator pedal, for selecting a maximum ground speed, still greater overall precision of vehicle control could be realized.
A control system for a transmission of the above-discussed kind should preferably have still additional capabilities. Variation of drive ratio is accomplished in part by changing displacements of fluid translating devices in the hydrostatic section of the transmission but when the limits of displacement change are reached it is necessary to upshift or downshift between fixed drive ratios in the mechanical sections of the transmission if infinite variation of the overall drive ratio of the transmission is to continue. To simplify the operator's task the control system should preferably accomplish this automatically.
Considering still another desirable capability for a control system of the kind discussed above, it has been pointed out that the objective of maintaining a constant engine speed can be accomplished primarily by utilizing a governed engine freed from any direct control by the operator's accelerator pedal. However, the ability of a governor to compensate for changes of load on an engine is subject to limits. Accordingly it is preferable that the control system include means for maintaining the constant engine speed in the presence of load increases greater than can be compensated for by the engine governor. Still further, it is advantageous if the control system automatically inhibits overly rapid vehicle deceleration upon sudden retraction or release of the accelerator pedal.